The invention comprises 10-(4-N-leucauramine - benzoyl) - 3 : 7 - bis - (dimethylamino)-phenothiazine of the formula FORM:0817564/IV (c)/1 and a method for making it by condensing Michlers hydrol (4 : 41-bis-(dimethylamino)-diphenylcarbinol) with 10 - (4 - aminobenzoyl) - 3 : 7 - bis - (dimethylamino) - phenothiazine of the formula FORM:0817564/IV (c)/2 in boiling aqueous methanol. An example is given. The substance is used in a coating for colourless transfer sheet material and turns blue on contact with acidic material. The Provisional Specification states that 10-(4-amino - benzoyl) - 3 : 7 - bis - (dimethylamino)-phenothiazine may be made by nitrating benzoyl - leuco - methylene blue, separating p-nitro derivative and reducing it to the corresponding amine.
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